1. Field of the Invention
The present invention relates to a page turning effect generating apparatus for video signals.
2. Description of the Prior Art
As methods of changing over a video image (video screen) from an image A to an image B, there are generally known dissolve, cut-out and cut-in, wipe and so on, while a page turning method is used as a special image change-over method.
The page turning method is performed such that an image A is first displayed, as shown in FIG. 1A, the image A is then gradually replaced by an image B as if a page of a book is being turned over, as shown in FIGS. 1B to 1D, and finally the image B is displayed as shown in FIG. 1E. In this event, a portion out of the image A being turned over which is outside the screen shown in FIG. 1D (shown by a phantom line) is not naturally displayed. Further, as shown in FIGS. 1B to 1D, a turned portion of the image A is displayed in reverse, and a bent portion formed by the page turning is displayed as being deformed into a cylindrical shape (non-linear).
In the explanation below, respective portions will be designated by the following names which are also shown in FIG. 2, as occasions arise:
Display screen D: An overall image displayed on a screen of a display;
Previous image A: An image which is to be erased by a page turn-over;
Remaining portion Z: A portion of the previous image A which is not yet erased;
Next image B: An image which is to appear by a page turn-over.
Reverse portion R: A portion of the previous image A which is being reversely displayed by a page turn-over;
Edge L: An edge for turning the previous image A (a border between the next image B and the reverse portion R);
Nonlinear portion N: A portion of the remaining portion Z and the reverse portion R which is deformed as being nonlinear or cylindrical in the vicinity of the edge L (a portion indicated by hatching); and
Hidden portion S: A portion of the previous image A which is hidden by the reverse portion R.
Incidentally, in order to obtain page turning effects as mentioned above, image data of the remaining portion Z, image data of the reverse portion R and the image data of the next image B are necessary. Among the image data, as to the next image B, when a scanning position on the displayed screen D arrives at the next image B, image data at that position is merely outputted as it is as image data on the next image B. Such processing may be performed likewise for the remaining portion Z except for the nonlinear portion N.
However, in the nonlinear portion N, it is necessary to modify the order of the image data of the previous image A to a nonlinear or cylindrical form. Also, in the reverse portion R except for the nonlinear portion N, it is necessary to modify the order of the image data of the previous image A, though in a linear form.
For this reason, image data necessary for the nonlinear portion N has been conventionally derived by linearly writing image data of the previous image A into a video memory as well as generating nonlinearly changing read address signals by a cylindrical address generating circuit.
The above configuration for deriving nonlinear image data, nevertheless, requires a very complicated and expensive cylindrical address generating circuit. Also, the configuration allows little freedom relative to the shape (apparent shape in cross-section) of the nonlinear portion N when the reverse portion R is turned over. Therefore, the hardware configuration must be modified for producing the nonlinear portion N in a shape other than a cylindrical shape, for example an involved shape.
Further, since read addresses are generated separately for the portions N and R and the portion Z, the address generating method is complicated, which results in reduced freedom relative to the image changing processing. The configuration becomes further complicated when performing other page turning effects, for example, transferring the edge L in a fan shape simultaneously with a page turn-over.
Furthermore, there is little compatibility of hardware with other page turning effect generating circuits, for example, a three dimensional rotation effect generating circuit.